An object of the invention is to provide a way of dispensing small measured volumes of a liquid additive from a reservoir at atmospheric pressure to a fermenter vessel under pressure with great accuracy. By "small" is meant quantities in the range 0.4 to 10 ml. and the pressure of the vessel may be of the order of 3 bar. The invention is particularly, but not exclusively, applicable to the dispensing of measured quantities of an additive, for example, a nutrient, to a fermenter vessel.